(1) Field of the Invention
This invention relates to an exposure intensity detecting system suitable for use in a copying machine, and more specifically to an exposure intensity detecting system which can achieve correct detection of the intensity of light to be used for the exposure of a photosensitive member of a copying machine without affecting the exposure of the photosensitive member.
(2) Description of the Prior Art
In order to obtain vivid duplicate marks or images of good quality by a copying machine, it is essential to maintain the degree of exposure of its photosensitive member at an optimum level. However, the output of a light-source lamp which is adapted to illuminate each original document tends to drop as time goes on. Furthermore, dirt, smear, smudge and the like deposit on the surfaces of lens, reflectors and the like which serve to project light from the light-source lamp onto the original document. The illuminance on the surface of the original document is thus reduced. In addition, the background color density varies from one original document to another. Accordingly, the degree of exposure of the photosensitive member changes to an undesirable level. As a result, the quality of the duplicate marks is reduced. With a view toward overcoming this problem, it has been proposed to detect the intensity of light to be irradiated onto an original document and then to control the intensity of light to be radiated from a light-source lamp in accordance with the detection signal. The need for this kind of control is high especially for high-grade or advanced models, because they are required to have such performance as forming duplicate marks of high quality.
In a copying machine, a lens system is generally employed to focus an optical image from an original document on a photosensitive layer on a rotary drum. Usually, an optical unit with this lens system incorporated therein has a large volume. For several reasons in which the above-mentioned large volume of such an optical unit is also contained, it is indeed difficult to secure a sufficient space for the installation of a photosensor. For this reason, such a photosensor has conventionally been provided, for example, by forming a slit in a reflector which is combined with its associated light-source lamp and disposing the photosensor behind the reflector in such a way that light strikes the photosensor after its passage through the slit. This method is however accompanied by such problems that the distribution of light to be projected onto an original document is adversely affected by the thus-formed slit and since the position of the slit varies slightly by the expansion of the reflector due to heat given off from its associated lamp, the intensity of light cannot be detected correctly. It is also difficult to machine a slit through a reflector which is in the form of a curved surface.